The present invention relates to methods and apparatus for the modification of recorded data and, more particularly, to a novel technique for the direct over-write modification of digital data stored in domains of a magnetic-optic recording media.
The concept of storing binary bits of information from a digital data stream in a sequential multiplicity of domains formed in magnetic storage media is well known. While many different types of magnetic media have been hitherto used, including plated wires, toroidal cores, tapes and the like, the particular high-information-density media of interest here is a thin film layer of a magneto-optic recording material. Magneto-optic recording materials are amorphous ferrimagnetic alloys usually including a rare earth in combination with a transition metal such as amorphous alloys of terbium cobalt (TbCo), gadolinium terbium cobalt (GdTbCo), and the like materials.
With magneto-optic materials, the binary value of a stored bit of information can be determined by analyzing the effect upon a polarized light beam reflected from the surface. Ferrimagnetic materials chosen to have a high coercivity at room temperatures and low coercivity at higher temperatures, can be "written" by heating a small region to have a net magnetization which is not only substantially perpendicular to the surface of the film but is also established in that direction parallel to the direction in which an external (bias) magnetic field was directed at the time when that particular region was heated and allowed to subsequently cool. It is also well known that the external field direction can be changed to encode the data to be stored. Previously stored information can be changed by re-heating the film region while an external bias magnetic field is presented in the desired (opposite) direction through the region. In such magneto-optic systems in which an external bias magnetic field is used to change the previously-stored information, the preferred recording material is a ferrimagnetic material with a compensation temperature (Tc) at about room temperature. However, the speed at which an external magnetic field can be made to reverse cannot presently be made as fast as is desired. Thus, although magneto-optic storage media, have demonstrated both (1) sufficient data density for the storage of gigabits of information on a disk and (2) short time for access to the previously stored data, they have not hitherto allowed randomly stored data to be modified at any speed even close to the speed at which stored data can be read from the storage disk.
For general use, data storage equipment should be capable of writing, reading and/or over-writing data at the same high rate. Accordingly, a method and apparatus by which to rapidly modify the data stored in microscopic recording regions of a magneto-optical recording medium is highly desirable.